This invention relates to the recovery of uranium from phosphate compounds and, more specifically, to the recovery of uranium from phosphoric acid produced by the acidulation of phosphate rock.
Wet-process phosphoric acid is produced by digesting phosphate rock with sulphuric acid. The chemical reaction forms phosphoric acid and calcium sulfate. The latter is filtered out, providing enormous quantities of gypsum, a waste product, which leaves an impure acid stream typically containing about 30% P.sub.2 O.sub.5. Most of the uranium in the original rock shows up in the 30% acid, and various extraction processes have been developed to extract this uranium. The 30% acid is generally evaporated to about 54% "merchant acid", which is either sold or used to manufacture a variety of products, chiefly fertilizers. The higher the acid concentration, the harder it is to extract the uranium. Therefore, the 30% stage is where the uranium extraction normally takes place. If the uranium is not extracted, it ends up as a minor impurity in the various end products.
A number of prior processes have been developed to recover the minor amounts of uranium contained in wet-process phosphoric acid. In many of these processes, any hexavalent uranium present in the wet-process acid is first reduced to the tetravalent state and then extracted by contacting the acid with an organic extractant which has a high extraction coefficient (Ea) for uranium in the tatravalent state. The coefficient of extraction (Ea) is a measure of the extraction power of a reagent and is defined as the ratio of the concentration of uranium in the organic phase to the concentration of uranium in the aqueous phase at equilibrium. The pregnant extractant is stripped with a suitable stripping solution, such as phosphoric acid, and the pregnant stripping solution treated to recover the uranium. For example, the pregnant stripping solution may be subjected to a second extraction step to obtain a uranium-rich extractant solution which can be further processed to recover uranium.
Alkylphenyl acid phosphate is a known organic extractant which has a high extraction coefficient for uranium in the tetravalent state. The alkylphenyl acid phosphate extractant is formed by reacting an alkylphenol with phosphoric oxide and comprises a mixture of mono- and di-(alkylphenyl) esters of orthophosphoric acid. The extractant is commercially available as the octylphenyl acid phosphate and is produced commercially at approximately a 1:1 mono- to di-ester mole ratio. While the extractant has a relatively high extraction coefficient for tetravalent uranium, the extractant is known to hydrolyze over extended periods of use to form primarily alkylphenol and phosphoric acid. The hydrolysis not only results in a loss of the extractant, but also causes the mono- to di-ester mole ratio to change since the mono-ester hydrolyzes more rapidly than the di-ester. The change in the mono- to di-ester mole ratio also is caused by the preferential loss of the mono-ester to the wet-process acid over extended periods of use as a result of the relatively higher solubility of the mono-ester in the acid compared to the di-ester. It has previously been reported that the change in the mono- to di-ester mole ratio does not significantly decrease the extraction coefficient in the di-ester mole fraction range of 0.5 to 0.88 based on the total mixed esters present in the extractant. However, as a result of the hydrolysis, the extraction coefficient will eventually decrease and the extractant will become increasingly less effective in extracting uranium from wet-process acid.
As disclosed in commonly assigned, copending application Ser. No. 88,152, filed Oct. 25, 1979, which is a continuation of Ser. No. 772,818, filed Feb. 28, 1977, now abandoned, for "PROCESS FOR EXTRACTING URANIUM FROM WET-PROCESS PHOSPHORIC ACID" by William M. Leaders et al, it may be desirable to add a phenol modifier such as an alkylphenol (e.g., octylphenol) to the extractant to eliminate the precipitation of the ferric salt of the mixed ester. Ferric iron is normally present in wet-process phosphoric acid and is detrimental to the extraction coefficient of the mixed ester. In addition, it will cause the precipitation of the ferric salt of the mixed ester. As disclosed in the aforementioned commonly assigned, copending application, the addition of about 1 to 10% by volume, preferably about 2 to 6% by volume, of a phenol modifier will substantially prevent the loss of the mixed ester as a result of precipitation. It has been found, however, that as a result of a loss of the extractant and an increase in the concentration of the alkylphenol in the extractant due to hydrolysis, the extraction coefficient is actually depressed.
In addition to the chemical changes which occur in the alkylphenyl acid phosphate extractant, physical changes also occur over extended periods of use. The physical changes include viscosity, specific gravity, and solids content increases. While the chemical changes manifest themselves in lower extraction coefficients, the physical changes cause increasingly more pronounced emulsion formations, flow control problems, and overall organic handling difficulties. Therefore, there exists a need in the art for a process to regenerate the extractant.
Accordingly, it is an object of the present invention to provide a process for regenerating an alkylphenyl acid phosphate extractant.
It also is an object of the present invention to provide a process for regenerating an alkylphenyl acid phosphate extractant which is efficient and economical.
It is a further object of the present invention to provide a process whereby the extraction coefficient of the extractant is maintained at an optimum level.